This invention relates generally to catheters, and more particularly to catheters having rotatable operative elements.
Currently, there exist rotating element catheters, which can be used by physicians to provide a diagnostic or therapeutic effect within the body tissue of a patient, e.g., ultrasonic imaging or artherectomy. A typical rotating element catheter includes a flexible drive cable that extends the length of the catheter body, terminating proximally in a motor drive unit. An operative element, e.g., an ultrasonic transducer or artherectomy blade, is distally mounted to the drive cable. Operation of the drive unit rotates the drive cable, which, in turn, rotates the operative element at high speeds to produce the desired diagnostic or therapeutic effect. Due to the nature of placing indiscriminately rotating elements inside a patient, there is always a risk that the rotating element could inadvertently damage tissue if the catheter is defective or mishandled.
For example, some ultrasonic imaging catheters can provide two-dimensional 360xc2x0 images along the length of a blood vessel by rotating an ultrasonic transducer at high speeds, while linearly moving the ultrasonic transducer in the distal direction relative to the catheter member. If the distal end of the catheter member is kinked, or otherwise formed into a tight curve, there exists the possibility, however so slight, that the rotating ultrasonic transducer could perforate through the catheter member and damage the surrounding tissue. This is caused, in part, by the fact that the drive unit is designed to maintain the speed of the transducer at a set level, accordingly increasing or decreasing the torque that is applied to the drive cable. In doing so, the drive unit does not discriminate between normal frictional loads, i.e., frictional loads caused by normal friction between the drive cable and catheter member, and abnormal friction loads, i.e., frictional loads caused by an abnormal circumstance, e.g., the boring of the transducer through the wall of the catheter member.
As a precaution, these types of ultrasonic imaging catheters are designed, such that the drive shaft fails if the torque required to rotate the ultrasonic transducer becomes too great. This design contemplates providing a circumferential space between the drive cable and the catheter member along a portion of the catheter, allowing the drive cable to wind or ball up within the space when the torque applied to the drive cable exceeds a critical magnitude. Presumably, such an excess in force will occur if the rotating ultrasonic transducer begins to perforate the catheter member, resulting in a failed drive cable, and preventing the ultrasonic transducer from further boring through the catheter member.
Typically, however, the drive shaft fails, not because the ultrasonic transducer is boring through the catheter member, but rather because the drive cable is subjected to excessive frictional forces. Such forces are often a result of having to route the catheter through the tortuous vasculature of a patient, forcing the drive cable to rotate through many curves. Any mishandling of the catheter while operating the motor drive unit, e.g., overtightening the touhy-borst valve through which the catheter is introduced into the patient, exacerbates this situation. Because the drive unit is designed to maintain the rotation of the ultrasonic transducer at a uniform speed, the motor drive unit increases the torque that is applied to the drive cable to compensate for any increase in frictional force, thereby risking failure of the drive cable. In fact, of all the failed ultrasonic imaging catheters returned to the assignee of this application, approximately seventy percent fail as a result of this phenomenon.
There thus remains a need to prevent premature failure of a drive cable within a catheter, while minimizing the potential risk of inadvertently damaging tissue by the rotating operative element distally mounted on the drive cable.
The present inventions are broadly directed to rotating element catheters and catheter assemblies that employ magnetic and ferrous members to prevent rotational energy from being transmitted from a motor drive unit to the catheter element under defined circumstances.
In accordance with a first aspect of the present inventions, a catheter assembly includes an elongate member in which there is disposed a rotatable catheter drive shaft, e.g., a flexible drive cable. The catheter drive shaft may have an operative element, e.g., an ultrasonic transducer or an artherectomy blade, distally mounted thereon for providing diagnostic or therapeutic functions to the physician. In the case of ultrasonic imaging, the elongate member can take the form of a telescoping guide sheath slidably disposed about an imaging core (i.e., the catheter drive shaft and ultrasonic transducer) to provide the physician with two-dimensional 360xc2x0 ultrasonic images of surrounding body tissue.
To control the rotation of the catheter drive shaft, the catheter assembly includes a driver member and a driven member, one of which is magnetic and the other of which is ferrous. The driven member is rotatably coupled (either directly or indirectly) to the proximal end of the catheter drive shaft. The driver member magnetically cooperates with the driven member, such that the driven and driver members are rotatably engaged with each other before the applied torque exceeds a critical magnitude, and rotatably disengaged with each other after the applied torque exceeds the critical magnitude. The driven and driver members are preferably located entirely within the catheter, e.g., in a proximal hub configured to interface with a motor drive unit, but a portion of the entirety of the driven and driver members can be located elsewhere, e.g., in the motor drive unit.
In the preferred embodiment, the magnetic member includes a plurality of permanent magnets, and the ferrous member includes a plurality of ferrous elements that are adjacent the permanent magnets. Although other spatial relationships can be used, the magnetic and ferrous members in the preferred embodiment are conveniently in a concentric relationship with each other. The driven and driver members may be located entirely within the catheter, e.g., in a proximal hub configured to interface with a motor drive unit.
In accordance with a second aspect of the present inventions, one of a driver member and a driven member includes a rigid receptacle having a cavity formed therein, and the other of the driver member and driven member includes a rigid member, which is disposed within the cavity of the rigid receptacle. The rigid member includes a plurality of outwardly facing permanent magnets disposed thereabout, and the rigid receptacle includes a plurality of inwardly facing ferrous elements disposed about the cavity. The plurality of permanent magnets are outwardly adjacent the permanent magnets.
In a preferred embodiment constructed in accordance with the second aspect of the present inventions, the permanent magnets, as well as the ferrous elements, are equally spaced from each other. In the preferred embodiment, the ferrous elements are inwardly extending, and the rigid receptacle includes a plurality of outwardly extending arcs between the ferrous elements, such that the magnetic attractive forces are concentrated at the ferrous elements. The ferrous elements and arcs can be formed, e.g., from a deformed inner surface of the rigid receptacle, or from curvilinear flanges mounted to the rigid receptacle. The rigid receptacle and member can be composed of a ferrous material to control the magnetic field produced by the magnets.
In accordance with a third aspect of the present inventions, one of a driver member and a driven member includes a rigid receptacle having a cavity formed therein, and the other of the driver member and driven member includes a rigid member, which is disposed within the cavity of the rigid receptacle. The rigid member includes a plurality of outwardly facing ferrous elements disposed thereabout, and the rigid receptacle includes a plurality of inwardly facing permanent magnets disposed about the cavity. The plurality of permanent magnets are outwardly adjacent the permanent magnets.
In the preferred embodiment, the permanent magnets, as well as the ferrous elements, are equally spaced from each other. In the preferred embodiment, the ferrous elements are outwardly extending, and the rigid receptacle includes a plurality of inwardly extending arcs between the ferrous elements, such that the magnetic attractive forces are concentrated at the ferrous elements. The ferrous elements and arcs can be formed, e.g., from a deformed outer surface of the rigid member, or from curvilinear flanges mounted to the rigid member. The rigid receptacle and member can be composed of a ferrous material to control the magnetic field produced by the magnets.